The gene that controls expression of the apolipoprotein ?4 allele, APOE4, is the greatest genetic risk factor for Alzheimer's disease (AD), increasing risk up to 15-fold compared to APOE3. Although this risk, associated with enhanced amyloid-? (A?) accumulation, was discovered over 20 years ago, it has rarely been the focus of therapeutic approaches; even less so, the critical link between female sex and the APOE4-induced risk for dementia and AD. Female (?) APOE4 carriers have a greater lifetime risk for developing AD, an increased rate of cognitive decline and accelerated accumulation of A? compared to male (?) APOE4 carriers, data consistent with observations in ? and ? familial AD (FAD) transgenic mice. Estradiol (E2) would appear to be key to this vulnerability: ablation of circulating E2 in pre-menopausal women by oophorectomy causes later cognitive deficits that are reversed by estrogen therapy (ET). The effects of ET after natural menopause remain controversial, particularly with regard to the ?critical window? hypothesis stating that ET is beneficial only in the early menopause window. A further concern is the unfavorable safety profile associated with ET resulting from the Women's Health Initiative studies, requiring the development of safer ET alternatives (alt-ET). The goal of this proposal is to determine the ability of ET and alt-ET to counteract the negative interaction of sex with APOE4 in the novel preclinical EFAD mouse model (expressing human APOE +FAD mutations), establishing both the preferred timing and APOE isoform selectivity of safe alt-ET for therapy or prophylaxis of AD. The EFAD mouse was developed to study the interaction between human h-APOE and AD pathology by introducing the h-APOE genotypes into 5xFAD mice: EFAD mice display advanced pathology in females vs. males and E4FAD vs. E3FAD; and preliminary EFAD data suggests that ET after ovariectomy (OVX) protects against cognitive deficits. Alt-ET to be studied are clinical selective estrogen receptor (ER) modulators (SERMs), raloxifene (Ral) and bazedoxifene (Baz); the clinical Baz/E2 combination; and selective estrogen mimics (SEMs) with attenuated gynecological effects. Aim 1: Establish the efficacy of ET (E2) treatment in OVX ?EFAD mice with respect to APOE genotype, measuring behavior/memory, apoE and A? biomarkers, and AD pathology. Aim 2: Establish the ?critical window? for ET in EFAD mice with respect to genotype. Aim 3: Test alt-ET in female EFAD mice as transformational AD therapeutics. Dosing will be defined by measuring brain concentrations related to in vitro Kd and EC50. To probe the roles of ER isoforms, mixed glial cultures from APOE-TR mice, will be studied using selective pharmacological ER-probes. In vitro biomarkers (apoE, A?, ABCA1, cytokines) will mirror in vivo biomarkers, allowing correlation with in vivo effects of ET and alt-ET, and development of an in vitro assay for future discovery and optimization of Alt-ET for AD.